Provide API for merging/stitching two paths together in the standard library

I saw some of the recent changes to the [ExecTainted.ql](https://github.com/github/codeql/blob/main/cpp/ql/src/Security/CWE/CWE-078/ExecTainted.ql) C++ query where we stitch two paths together. I have needed this functionality before for my own queries, and after a lot of effort hacked something similar together to work for me. But given that this functionality is now used in a query in the default suite, can we integrate it into the standard library so others can more easily use it?

cc: @rdmarsh2